1. Field of the Invention
The present invention is directed to an apparatus for generating optical waves which are polarized orthogonally relative to one another from an incoming optical wave constructed in integrated optical technology.
2. Description of the Prior Art
Many alternatives are known for the realization of coherent optical communications systems. A reception system making use of a technique known as polarization quadrature reception avoids the problem of fluctuating polarization direction of the incoming signal, which is supplied by a suitable communications system link, by resolving the incoming signal into two sub-signals polarized orthogonally relative to each other, for example TE-signals and TM-signals. This is accomplished in the receiver with the sub-signals then being separately superimposed with the local oscillator signal.
Heretofore such receivers have been constructed for both free beam propagation and fiber optics beam propagation. The former has the disadvantage that a transition from a guided optical wave into a freely propagating wave is necessary. The latter involves difficulties in manufacture as well as difficulties related to the sensitivity of the component properties to temperature fluctuations or to bending of the fiber.
Previous attempts at constructing an integrated optical receiver of this type have only been generally described in the literature (see, for example, Electon. Lett. 23 (1987), pages 614-616). The polarization-dependent degree of coupling of a directional coupler is exploited in this proposal and the directional coupler is dimensioned such that, for example, the TE-polarized optical wave completely couples to a second optical waveguide of the directional coupler, whereas the TM-polarized wave remains entirely in the first waveguide of the coupler. Minute fluctuations in the manufacturing process have an unfavorable influence on the functionability of the components, and produce signal degrading crosstalk. The aforementioned proposal seeks to alleviate these problems by providing an electrode in the directional coupler to at least permit control of the coupling behavior of the TM-polarized wave. There is no possibility provided for controlling the coupling behavior of the TE-polarized wave.